Oscillators are circuits for converting dc power into a periodic waveform or signal. Conventional RC oscillators advantageously furnish a low-cost timing source and allow for generation of variable frequencies by changing the resistance R, or capacitance C. Furthermore, conventional RC oscillators advantageously avoid the use of inductors, which are difficult to fabricate on integrated circuits.
However, various disadvantages and limitations are associated with RC oscillators. These disadvantages and limitations greatly restrict the applications into which the RC oscillator can be utilized. Typically, conventional RC oscillators are limited to utilization as low-frequency, low-precision signal sources such as tone generators, alarms or flashing indicators. Even in applications which require only moderate frequency accuracy of one to ten percent, more costly crystal oscillators traditionally replace RC-oscillator circuits. For example, RC oscillators are typically restricted to low-frequency applications in circuits in which the operating frequency is less than 5 Mhz. For applications requiring operating frequency error to be less than 20%, the operating frequency is restricted to less than 2 MHz. Furthermore, conventional RC oscillators are often very inaccurate, having a frequency error of greater than 15%. Utilization of on-chip components further degrades oscillator accuracy. In addition, conventional RC oscillator circuits are notoriously sensitive to process and temperature variations.
The performance of a conventional RC oscillator, shown in FIG. 1, is limited by intrinsic delays within the circuit and by parasitic resistances and capacitances which degrade the value of the frequency selective elements R and C. The RC oscillator includes a first inverter INV1, a second inverter INV2, a resistor R and a capacitor C. FIG. 2 illustrates the RC oscillator circuit shown in FIG. 1 but also includes the intrinsic delays and parasitic circuit elements that degrade the performance of the oscillator. The intrinsic delays include delays through the amplifier stages of the circuit, specifically delay 1 through inverter INV1 and delay 2 through inverter INV2. These delays increase sensitivity of operating frequency variability to temperature variations because carder mobility decreases as temperature increases. The parasitic circuit elements include the parasitic input capacitance of the integrated circuit package (Cin1, Cin2 and Cout2) and the input stage. The RC oscillator circuit shown in FIG. 2 also shows the effective series resistance of the output stage (Rout1 and Rout2). Table I provides a summary of errors which arise due to intrinsic delays and parasitic effects.
TABLE I ______________________________________ TYPICAL ERRORS OF CONVENTIONAL RC-OSCILLATORS Cosc = 500 pf, Rosc = 1/2 foscCosc VALUE 0.1 MHz 0.5 MHz 1.0 MHz 5.0 MHz ______________________________________ delay 1 2.5 ns 1E-3 5E-3 1E-2 5E-2 delay 2 2.5 ns 1E-3 5E-3 1E-2 5E-2 Rout1 .times. 100 ps 2E-5 1E-4 2E-4 1E-3 Cin2 Rout2 .times. 100 ps 2E-5 1E-4 2E-4 1E-3 Cout2 Cin1/Cosc 2E-2 2E-2 2E-2 2E-2 2E-2 Rout1/ -- 1E-3 5E-3 1E-2 5E-2 Rosc Rout2/ -- 1E-2 5E-3 1E-2 5E-2 Rosc Oscillator 2.4% 4.02% 6.04% 22.2% Total 3.4% 5.02% 7.04% 23.2% ______________________________________ NOTES: Total includes 0.5% tolerance for both Rosc and Cosc.
Generally, errors resulting from the intrinsic delays and parasitic effects increase greatly with the operating frequency of the RC oscillator circuit so that conventional RC circuits are substantially limited to applications in which the operating frequency is less than about 1 MHz. Errors in operating frequency due to delays in the circuit are non-trivial whenever operating frequencies are higher than 1 MHz.
One method for avoiding parasitic effects is to use the integrated circuit chip dielectric to replace the capacitor C because a smaller capacitance value allows more suitable resistances of the resistor R. However, typical process variations in dielectric thickness result in substantial frequency variability. Application of trim to reduce the effects of dielectric thickness variation increases the die cost.
Various techniques have typically been employed to improve performance of conventional RC oscillators. However, these techniques have often had self-defeating results. Numerous difficulties arise while attempting to obtain high precision results for RC oscillators operating at frequencies above 0.5 MHz.
For example, the external capacitor Cosc of the RC oscillator must be sufficiently large to overpower the parasitic package and board capacitance. However, for a capacitor Cosc this large, the external resistor Rosc must be sufficiently small to charge and discharge the capacitor Cosc. A resistor Rosc this small disadvantageously escalates the power supply current. In addition, while the external resistance of resistor Rosc must be sufficiently small to charge and discharge the capacitor Cosc, it must also be much larger than the effective series resistance of the output stage driving the RC network. It is difficult to resolve these conflicting constraints on the size of the external resistor Rosc.
Furthermore, the intrinsic delay of the oscillator circuit must be much smaller than the period of oscillation. Also the W/L ratio of transistors in the output driver must be large to suitably reduce the output impedance of the driver. However, a large W/L ratio increases input capacitance and consequently the delay through the stage previous to the output driver, which then increases the intrinsic delay of the oscillator circuit. In addition, increasing the W/L ratio of transistors in the output driver elevates crossover current of the output stage, producing ground bounce and other adverse effects.
The intrinsic delay of the oscillator may also be reduced by increasing the W/L ratio of transistors in the input stage of the oscillator. However, increasing the W/L ratio disadvantageously increases the parasitic capacitance of the input stage, which disturbs the design value of capacitance C.
It has been observed that precision better than 4% at operating frequencies above 1 MHz or accuracy better than 8% at operating frequencies above 2 MHz is either unattainable or impractical utilizing conventional techniques for improving RC oscillator performance.
Due to the aforementioned drawbacks, conventional approaches to an RC oscillator circuit do not provide for a high performance, cell-based CMOS RC oscillator for usage in ASIC applications. Therefore a new approach is necessary.